feat(propose): --shell-parent points a heading-only section at the ancestor holding its text

A plan entry whose span holds its heading alone gets `parent_id` naming the
nearest preceding entry at a smaller level whose own span holds text, passing
over an ancestor that is empty too; the door writes the existing `parent:`
key. Nothing is copied and no boundary moves. The rule reads the plan's level
and order, never the row. Off by default: `okf consume` reads no `parent`
key, so no payload ranks differently, while the flag moves the bytes of every
bundle holding a heading-only section.

Measured before building on one 2 761-concept process code: 710 concepts are
heading-only; the level route names the ancestor the document's own <sec>
nesting names on 708 of 710 (two sit at depth 7, clipped to 6, and point one
level too high), where reading section numbers gets 686 (`12` begins with
`1`); 35 have no ancestor holding text and get none.

The red test expected only concept files to move. The index is a projection
of the frontmatter and shows the key as a facet, so the test now holds both;
the facet renders a segment id as unresolved (`p1?`), because `structure`
reads `parent` as a document number -- named in README, CLAUDE.md and the
CHANGELOG, not repaired here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_shell_parent.py

@@ -23,6 +23,7 @@ sentence from any source.
 
 from __future__ import annotations
 
+import re
 from pathlib import Path
 
 from llm_ingestion_okf import cli, extract, propose
@@ -167,13 +168,16 @@ def test_a_build_without_the_flag_is_the_build_with_the_opt_out(tmp_path: Path)
     assert files == sorted(p.relative_to(opted_out) for p in opted_out.rglob("*") if p.is_file())
     for relative in files:
         assert (default / relative).read_bytes() == (opted_out / relative).read_bytes()
-    # The flag adds exactly one `parent:` line to each shell with an ancestor.
+    # The flag adds one `parent:` line to each shell with an ancestor, and the
+    # index -- a projection of the frontmatter -- shows the same key as a facet
+    # on that shell's entry. Nothing else moves.
     added = 0
     for relative in files:
         before = (default / relative).read_text(encoding="utf-8").split("\n")
         after = (flagged / relative).read_text(encoding="utf-8").split("\n")
-        extra = [line for line in after if line not in before]
-        assert all(line.startswith("parent: ") for line in extra)
+        if relative.name == "index.md":
+            assert [re.sub(r"parent: p\d+\?; ", "", line) for line in after] == before
+            continue
         assert [line for line in after if not line.startswith("parent: ")] == before
-        added += len(extra)
+        added += sum(1 for line in after if line.startswith("parent: "))
     assert added == sum(1 for ancestor in EXPECTED.values() if ancestor is not None)